The purpose of this R21 application is to test the hypothesis that the basis of the capacity of environmental mycobacteria [EM] to interfere with the efficacy of prior BCG vaccination is due to the induction of a subset of CD4 T cells called regulatory T cells. We have established a highly reproducible model in which mice are vaccinated with BCG, then exposed for six months to dead M.avium, then have shown that resistance is ablated or substantially diminished if the animal is then challenged with M.tuberculosis. Our preliminary studies now suggest that this may be due to the capacity of the EM to induce regulatory T cells, and so to test this hypothesis we will measure the accumulation of these cells in the lungs after challenge, deplete them to see if this restores resistance, and determine if their secretion of the cytokine IL-10 is the key basis of interference. Finally, we wil test this hypothesis under realistic conditions, by challenging mice with new emerging W-Beijing strains that are themselves potent inducers of regulatory T cells. These studies have an important practical basis, given that new BCG based vaccines are about to be tested in people in which EM exposure is known to exist, and where virulent W-Beijing strains abound.